My prior U.S. Pat. Nos. 4,547,970 and 4,553,337 disclose and claim thread gauges for measuring the pitch diameter of cylindrical or non-tapered threads. While those gauges have proven to be extremely effective and useful for that particular type of thread configuration, they cannot be used with accuracy for parts or objects in which the pitch line of the threads forms a tapering cone instead of a straight cylinder.
Accordingly, one important object of the present invention is to provide a thread gauge which permits the user to accurately and conveniently measure the pitch diameter of an object having tapered threads. Furthermore, it is an important object to be able to measure such tapered threads whether they are on the outside or inside the object being checked.
Additionally, an important object of the present invention is to provide a thread gauge for tapered threads which carries on the general principles of use and operation provided in my earlier patents as above identified.
Still further, it is an important object of this invention to provide a tapered thread gauge which is particularly well suited for use in connection with a conventional vernier caliper, but which can also be utilized in connection with an inside micrometer without departing from the principles of operation and construction herein set forth.
In carrying out the foregoing and other objects, the present invention contemplates using a pair of thread anvils, as in my prior patents, which match the thread pitch and configuration of the part being measured. Such anvils are attached to the spreadable arms or supports of a vernier caliper, or to the opposite ends of an inside micrometer, to essentially adapt that device to take an accurate measurement of the threads on the object to be measured. The problem with measuring threads of a tapered configuration is that the pitch diameter is constantly changing over the length of the threaded portion of the object, due to its conical shape. While with straight cylinder threads it makes no difference where the measurement is taken along the length of the threaded portion, in a tapered configuration the linear point at which the measurement is taken is of critical importance.
Standard protocol requires that the pitch diameter of a tapered thread be called out at the intersection of the thread pitch cone and the end face of the object being measured. Due to the helical nature of threads, however, the exposed end face of the thread is set back or recessed away from the actual end surface of the object by a significant distance at a point that is 180.degree. around the rim from the starting point of the thread. Consequently, there are no two diametrically opposed physical points exactly in the same plane at the end of the object that can be engaged by a measuring tool to determine the pitch diameter.
In the gauge of the present invention, by designing the thread anvils so their lines of threads are caused to taper or slope at the same angle as the tapered thread on the part to be measured, such anvils will mesh properly with the threads of the part when the anvils are applied to the part. Furthermore, since the thread anvils will, in effect, have their own tapering thread cone effectively defined between the two anvils, the intersection point between that pitch cone and the end face of the anvils can be precisely located at known distances from reference surfaces on the gauge. Consequently, by then registering those pitch cone intersection points of the thread anvils with the corresponding conical intersection points of the part to be measured, the intersection points on the part will likewise come to be located at the known distances from the reference surfaces of the tool. When these known distances are then added to or subtracted from the displayed distance reading on the measuring tool for the space between its reference surfaces, the resulting figure is the distance between the two diametrically opposed cone intersection points on the part, i.e., the pitch diameter of the part.
To make sure that the thread anvils of the present invention become located on the tapered thread of the object at the proper linear position, each of the anvils is provided with a special, overhanging stop finger that projects out beyond the thread line of the anvil into position to overlie the proximal end face of the object being measured. By abutting such end face, or nearly so, with the stop fingers, the imaginary cone intersection points on the thread anvils are forced into registration with the corresponding intersection points on the object being measured, to yield an accurate measurement.
In order to permit the two anvils on opposite sides of the object to be perfectly engaged with the threads on the object, it may be necessary for slight linear shifting or adjustment of the thread anvils to occur. Accordingly, the tapered thread anvils of the present invention are provided with ball couplings that join the anvils with the measuring tool itself in essentially the same manner as set forth in my prior '970 and '337 patents. Such ball couplings allow the anvils to shift linearly through a limited amount of travel as may be necessary to best seat the anvils on the object, while also permitting a certain amount of in-and-out rocking movement to likewise assure proper seating. It will be noted that this particular ball coupling arrangement also allows the anvils of the present invention to be interchangeably attached not only to vernier calipers, but also to inside micrometers, notwithstanding the constructional differences between those two types of tools.
By providing two different types of releasable connectors for coupling the mounting balls and their anvils to the leg supports of the caliper, the caliper may be set up for checking either externally threaded parts or internally parts, as the case may be. Notwithstanding this type of flexibility and adjustment, accurate measurements are assured.